This invention relates to ferrite resonators. More particularly, this invention relates to self biased ferrite resonators composed of hexagonal ferrites and to a process for their fabrication.
Single crystal hexagonal ferrites are characterized by having high uniaxial anisotropies combined with low loss characteristics which makes them especially valuable for use in microwave and millimeter-wave resonators. Generally, a ferrite resonator must be biased by an external magnetic field. The bias field serves two functions: to saturate the magnetization and to set the center frequency. However, the large anistropy field exhibited by hexogonal ferrites permits high frequency resonant devices to be operated with only a very modest external magnetic bias field.
Unfortunately, it is very difficult to prepare high quality device material by conventional methods that is capable of being maintained in a metastable state of magnetic saturation with zero, or near zero, applied field. This is especially true when growing ferrites in single crystal form. Single crystals, rather than ceramic type ferrites, provide the lowest losses and, therefore, the narrowest resonance line widths.
The growth of hexagonal ferrite single crystals has received considerable attention with a wide ranging research effort generated in an attempt to grow high quality crystals. The difficulties in growing big quality material arise both from intrinsic characteristics of the hexagonal ferrites and from limitations of the crystal growth methods. For use in devices, bulk crystals must have uniform composition and be free of defects. The crystals must be fabricated into special shapes, such as spheres or rods, having microscopically smooth surfaces.
Growth techniques which have proved to be somewhat successful are the flux methods, either by slow cooling with spontaneous multiple nucleation or by top-seeding. These methods are similar to those used commercially to grow YIG (yttrium iron garnet) ferrite bulk crystals for microwave devices. However, the hexagonal ferrites are not as easy to work with as YIG. It is much more difficult to grow homogeneous, high quality single crystals of hexagonal ferrites and it is considerably more difficult to fabricate these crystals into the well polished configurations needed in devices. Another method which has achieved an even greater degree of success is the isothermal dipping method of liquid phase epitaxy, otherwise referred to as the LPE method. This method resembles the top seeding method and is basically a flux or solution growth technique.
In order to achieve the objectives of this invention and overcome the problems encountered in previous attempts at growing high quality hexagonal ferrite single crystals, it was found necessary to provide very carefully controlled LPE growth conditions and carefully preselect specific processing parameters. With proper selection and careful control, single crystal ferrites could be grown that exhibited the necessary strong uniaxial anisotropy required for the resonators contemplated by this invention.